FIG. 4 is a cross-sectional view of a conventional electro-acoustic transducer disclosed in Japanese Patent Laid-Open Publication No. 2000-153231. FIG. 5 is an enlarged cross-sectional view of the transducer.
Diaphragm 1 is joined to frame 2 having a resin-molded periphery. An end of voice coil 3 is bonded to diaphragm 1. Upper plate 6 is bonded on magnet 5 bonded on yoke 4. Magnetic circuit 7 includes yoke 4, magnet 5, and upper plate 6. The other end of voice coil 3 is positioned untouched at magnetic gap 8 between yoke 4 and upper plate 6. Respective ends of suspensions 9a and 9b mounted on top and bottom surfaces of yoke 4, respectively are joined and suspended to frame 2. Suspension 9a and 9b are incorporated with resin caps 9c provided at respective ends thereof by outsert molding. Resin caps 9c are bonded to frame 2 and join suspensions 9a and 9b to frame 2. Yoke 4 and suspensions 9a and 9b are joined together by laser welding. Cover 2a is joined to frame 2 and covers an outer periphery of yoke 4.
An operation of the conventional electro-mechanical acoustic-transducer will be described. A signal is input to voice coil 3. If a frequency of the input signal matches a mechanical resonance frequency of a vibrator composed of suspensions 9a, 9b and magnetic circuit 7, magnetic circuit 7 resonates and vibrates up and down intensely, thus causing a user of a portable communication device including the electro-acoustic transducer to feel the vibration. If the frequency of the input signal matches a mechanical resonance frequency of a tone generator composed of diaphragm 1 and voice coil 3, diaphragm 1 resonates and vibrates up and down intensely, causing the user of the portable communication device to notice the vibration with sound.
The resonance frequency of magnetic circuit 7 may be set to approximately 100 Hz, and the resonance frequency of the tone generator may be set to approximately 1 kHz. The frequency of the input signal is selected, and the electro-acoustic transducer allows the user to feel the vibration by contact and to notice the vibration by tone.
If the frequency of the input signal matches the resonance frequency of magnetic circuit 7, magnetic circuit 7 resonates and vibrates intensely. However, vibration of diaphragm 1 that is generated as a reaction of the vibration of magnetic circuit 7 does not have a frequency high enough to enable users to notice the vibration by sound.
If the frequency of the input signal matches a mechanical resonance frequency of the tone generator composed of diaphragm 1 and voice coil 3, diaphragm 1 resonates and vibrates intensely, enabling the user to notice the vibration with sound. However, since magnetic circuit 7 does not follow the frequency of the input signal which is high due to a large mass of magnetic circuit 7, the vibration of magnetic circuit 7 that is generated as a reaction of the vibration of diaphragm 1 cannot have a large amplitude.
In the conventional electro-acoustic transducer, a surface of yoke 4 is provided with Ni plating and Sn plating to secure enough joinitng strength in vibrating components. Suspension 9a and 9b are welded to join to yoke 4 by laser beam. The production process, therefore, is under complicated controls to restrain possible fluctuations of the jointing strength due to fluctuations of the thickness of the metal plating or intensity of the laser beam.